Clarify blocksonly whitelistforcerelay test

Summary:

As discussed in https://github.com/bitcoin/bitcoin/issues/19943, this
test may be a bit misleading to newcomers.

We underscore the fact that our peer needs to run a modified version of
Bitcoin ABC to actually relay transactions to a blocksonly node and
benefit from the whitelistforcerelay parameter.

This is a backport of core#19963

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10301